1. Field of the Invention
The present invention relates to a conductive polymer suspension and a method for producing the same, a conductive polymer material, an electrolytic capacitor, and a solid electrolytic capacitor and a method for producing the same.
2. Description of the Related Art
Conductive organic materials are used for electrodes of capacitors, electrodes of dye-sensitized solar cells and electrodes of electroluminescence displays and the like. As such conductive organic materials, those conductive polymers obtained by polymerizing pyrrole, thiophene, aniline and the like are known.
Such conductive polymers are generally supplied as a dispersion (suspension) or a solution in an aqueous solvent or as a solution in an organic solvent, which is used as a conductive polymer material by removing the solvent at the time of use. However, even when the types of the conductive polymers are the same, the physical properties of the obtained conductive polymer materials are different from each other depending on the conditions of the dispersions; and hence the methods for producing such dispersions have hitherto been studied in various manners.
Japanese Patent Application Laid-Open No. 07-90060 discloses a solution (dispersion) of polythiophene and a method for producing the solution, and a technique of application of the solution (dispersion) to an antistatic treatment of a plastic molded article. The dispersion of polythiophene includes, as the dispersion medium, water or a mixture of a water-miscible organic solvent with water, a polythiophene formed of a structural unit of 3,4-dialkoxythiophene and a polyanion derived from polystyrenesulfonic acid having a molecular weight within a range from 2,000 to 500,000. The polythiophene is obtained by chemical oxidative polymerization, in the presence of the polyanion, of polystyrenesulfonic acid having a molecular weight within a range from 2,000 to 500,000. It is described that a transparent antistatic film can be formed in this manner.
Japanese Patent Application Laid-Open No. 2004-59666 discloses a water dispersion of a complex between poly(3,4-dialkoxythiophene) and a polyanion and a method for producing the water dispersion, and a technique related to a coating composition containing the water dispersion and a coated substrate having a transparent conductive film formed by applying the composition. The water dispersion is obtained by polymerizing 3,4-dialkoxythiophene in an aqueous solvent by using peroxodisulfuric acid as an oxidant in the presence of a polyanion. Alternatively, the water dispersion is obtained by chemical oxidative polymerization of 3,4-dialkoxythiophene in an aqueous solvent by using an oxidant in the presence of a polyanion under the pH of the reaction solution lowered by adding an acid selected from the group consisting of water-soluble inorganic acids and organic acids. It is described that a conductive thin film excellent in transparency can be formed in this manner.
However, by the methods described in Japanese Patent Application Laid-Open Nos. 07-90060 and 2004-59666, specifically by a method which conducts the chemical oxidative polymerization of 3,4-dialkoxythiophene in one stage in the presence of the polyanion acting as a dopant, it is difficult to obtain a polythiophene solution including fine particles because the molecular chain length of the polyanion is large. Further, the control of the doping rate of the polyanion is difficult. The molecular weight of the polystyrenesulfonic acid (polyanion) used in Japanese Patent Application Laid-Open Nos. 07-90060 and 2004-59666 is set at 2,000 or more, and hence the weight average molecular weight of the polystyrenesulfonic acid is naturally larger than 2,000 and in general, probably 10000 or more. The undoped polyanion, namely, the polyanion making no contribution to the conductivity comes to be present in an excessive amount, and thus, the methods are far from being satisfactory methods as the production method for obtaining a conductive polymer material having a higher conductivity.
The conductive polymer film obtained by the method described in Japanese Patent Application Laid-Open No. 7-90060 has a sufficient conductivity as an antistatic material, but hardly attains a conductivity sufficiently satisfying the requirement for lowering the ESR, for example, when used as a solid electrolyte in a capacitor. In other words, the surface resistivity of an antistatic film is generally classified to fall within a range from 105 to 1014Ω/□; when the conductivity is too high, there is a possibility of occurrence of an intense electrostatic discharge, and hence it is understood that the antistatic film does not have a conductivity so high as to rapidly dissipate the static charge of an charged object. Therefore, a material usable as an antistatic film is high in resistance as the solid electrolyte for use in capacitors and hence cannot satisfy the requirement of low resistance. Additionally, a capacitor including a solid electrolyte that contains excessive polyanions has a drawback of poor reliability, in particular, poor properties in a high humidity atmosphere.